I have written a function, setParagraphToRichText which accepts a GoogleAppsScript.Document.Paragraph and a string. The string is a stringified JSON blob that defines the text and attributes of a given paragraph.

/**
 *
 *
 * @param {GoogleAppsScript.Document.Paragraph} paragraph
 * @param {string} jsonString
 */
function setParagraphToRichText(paragraph, jsonString) {
  const text = paragraph.editAsText();